City Lab: The 21st time was the charm. After two decades of trying, the UNs climate change institution locked down international consensus from rich and poor nations alike to fundamentally change the ways humans power their activities. Almost 200 countries committed to voluntary greenhouse gas reduction targets and pledged to revisit them at regular intervals to limit global warming to well below 2 degrees Celsius. Its a deal that knows it isnt perfect, and takes steps to get better with time. And just...